1. Field of the Invention
The present invention relates to magnetoresistance transducers and is applicable in particular to the reading of data on multitrack magnetic record carriers such as rigid or flexible magnetic discs and magnetic tapes in which the data density is very high.
2. Description of the Prior Art
It is known that magnetic discs carry data on circular concentric recording tracks which have a radial width no greater than a few hundredths of a millimeter and commonly cover the greater part of both surfaces of the disc. Magnetic tapes, on the other hand, commonly carry data on tracks parallel to the tape length.
As a rule, a sequence of magnetic data recorded on the tracks of a disc or tape appears in the form of a succession of small magnetic areas referred to as "elementary areas" which are distributed throughout the length of the track and have magnetic inductions of identical modulus and opposed direction.
Longitudinal or linear density denotes the number of data per unit of length measured along the circumference of a track in the case of a magnetic disc, or along the tape length in the case of a magnetic tape. Radial data density (in the case of a magnetic disc) denotes the number of recording tracks per unit of length measured along the diameter of the disc.
The present trend in the development of magnetic discs is to increase the linear density as well as the radial density of the data.
The means which make it possible either to record data on discs or tapes, or to read recorded data therefrom or finally to perform one or the other of these two functions are referred to as "magnetic transduction devices", or more singly magnetic transducers. As a rule, one or more transducers are associated with a given record carrier which is driven so as to travel past and in front of the transducer.
In existing current practice, increasingly frequent use is made of transducers comprising one or more magnetoresistances, which are referred to under the more specific title of "magnetoresistance transducers" for reading data on discs or tapes. Magnetoresistance transducers are used in particular to read magnetic discs of very high linear and/or radial data density.
It will be recalled that a magnetoresistance is an element formed by magnetic material of which the electrical resistance R varies as a function of the magnetic field to which this element is exposed.
In present practice, these magnetoresistances are electrical resistances having the form of thin layers or films of very small thickness of which the length very greatly exceeds the width. The term "thin layer" as used herein denotes a layer having a thickness in the range of a few hundred Angstroms to a few microns. These magnetoresistances are frequently deposited on a substrate of electrically insulating material by well known deposition techniques.
Consider such a measuring magnetoresistance R connected to the terminals of a current generator which delivers a current having the intensity I flowing in the direction of its length. It is assumed that it appertains to a magnetoresistance transducer associated with a magnetic record carrier and that the transducer is situated at a distance from the carrier which is very small or even zero.
When each of the elementary magnetic areas passes before the transducer, the magnetic leak current H.sub.f generated by these areas close to the surface of the carrier causes a variation .DELTA.R of its resistance, giving a variation .DELTA.V=I.times..DELTA.R at its terminals, which yields: .DELTA.V/V=.DELTA.R/R, .DELTA.R/R being referred to as the "magnetoresistance coefficient". This coefficient is normally of the order of 2%.
The electrical signal collected at the terminals of a magnetoresistance is solely a function of the value of the magnetic field H.sub.f to which this magnetoresistance is exposed. For this reason, it has an amplitude unaffected by the speed of the record carrier in front of which is situated the magnetoresistance.
It will be recalled that "initial magnetic permeability of a magnetic material" denotes the ratio (B/H) between the magnetic field induction and the magnetic field itself when B and H are close to zero, and this on the first magnetisation curve. The first magnetosation curve is the curve showing the variation of B as a function of the field H when the magnetoresistance is exposed to a magnetic magnetisation field, and this starting from an initial magnetic state of the material defined by B and H being close to zero. In other words, the initial magnetic permeability of the material is equal to the slope of the first magnetisation curve close to the point at which B=0 and H=0.
It will also be recalled on the other hand that a magnetically anisotropic material positioned in a plane, having a thickness much smaller than its length and also its width, has in said plane two preferential directions of magnetization, commonly perpendicular to each other. One of these is referred to the "direction of easy magnetization". The other is referred to as the "direction of difficult magnetization". The initial permeability of the material in the direction of difficult magnetization greatly exceeds the initial permeability of the material in the direction of easy magnetization. The term "anisotropy field" H.sub.k is used to denote the total value of the magnetic field H which acts on any point of the material in its direction of difficult magnetization and from which saturation is obtained at this point in this direction.
The magnetoresistance utilized are commonly produced from a magnetically anisotropic material, for example an iron-nickel alloy (18% or iron, 82% of nickel). Their axis of easy magnetization is parallel to the direction of the current I and to their length, whereas their axis of difficult magnetization extends perpendicular to the former. The position of the one (of the) magnetoresistance(s) of a magnetoresistance transducer with respect to the record carrier associated with it (then), is such that it is exposed to the component of leakage field of the elementary areas which is parallel to its or their axis or axes of difficult magnetization, which is itself perpendicular to the surface of the carrier. When a magnetoresistance is not exposed to any magnetic field, it is said that it is inactive. In this case, the magnetization, that is to say the magnetic induction within the magnetoresistance, has the same direction as the axis of easy magnetization.
It can be shown that the efficiency or sensitivity of a magnetoresistance formed from an anisotropic magnetic material may be increased, that is to say the voltage of its output signal as a function of the magnetic field applied to it may be increased, by exposing the same to a magnetic polarizing field H.sub.pol parallel to its axis of difficult magnetization, as described in the French Patent No. 2165206 filed on Dec. 22, 1971 by Compagnie Internationale pour l'Informatique, under the title "Improved magnetoresistances and electromagnetic transducer incorporating same".
The value of the polarizing field H.sub.pol is selected in such a manner that the magnetization in the magnetoresistance is turned through an angle .theta. preferable close to 45.degree.. In this case, it can be shown that the efficiency of the magnetoresistance is a maximum, that is to say, that a given variation .DELTA.H of the magnetic field to which it is exposed (other than the field H.sub.pol) corresponds to a maximum variation of its resistance and hence of its output voltage. Moreover, it is possible to determine the direction of the magnetic field (or else the direction of the magnetic flux to which the magnetoresistance is exposed), which is not the case unless the magnetoresistance is polarized.
In existing practice, magnetoresistance transducers often comprise two parallel magnetoresistance elements (that is to say, their lengths and widths are parallel) separated by a distance of the order of a tenth or a micron. The distance separating them is at all events substantially or very much smaller than the length of the elementary magnetic carrier, so that these magnetoresistance are exposed to the same component of the leakage field, namely that which is produced by the area before which they are positioned.
The two magnetoresistance elements are each polarized to a value of the order of 45.degree. (in absolute value), their magnetizations then being at 90.degree. to each other as explained in the French Patent No. 2248566 filed by Compagnie Internationale pour l'Informatique on the Oct. 23, 1973 under the title "Improved electromagnetic transducer". The output signal .DELTA.v.sub.1 of the first magnetoresistance element is transmitted to a first input terminal of a differential amplifier, whereas the output signal .DELTA.v.sub.2 supplied by the second magnetoresistance element is transmitted to a second input terminal of the same differential amplifier. Since .DELTA.v.sub.1 is substantially equal to -.DELTA.v.sub.2, a signal proportional to 2.times..DELTA.v.sub.1 is collected at the output terminal of the differential amplifier.
It is equally demonstrable that the utilization of a differential amplifier renders it possible to establish a ratio between the useful signal, that is to say the signal proportional to v.sub.1, and the noise signal, that is to say the signal/noise ratio S/B. As a matter of fact, it is demonstrable that if B.sub.1 is the noise signal transmitted to the first input terminal of the differential amplifier, and if B.sub.2 is the noise signal transmitted to the second input terminal of the same amplifier, B.sub.1 and B.sub.2 have the same sign. As a result, a noise signal B proportional to B.sub.1 -B.sub.2, that is to say a very weak noise signal, is collected at the output terminal of the differential amplifier. It will be recalled that the noise signal is caused in particular by thermal disturbance in the magnetoresistance, and equally by all the magnetic fields other than the magnetic leakage field generated by the area opposite to which are positioned the two magnetoresistances.
As described in U.S. patent applications Ser. Nos. 242,923 and 242,924, filed on Mar. 12, 1981 in the name of Jean-Pierre Lazzari et al and assigned to Compagnie Internationale pour L'Informatique Cii-Honeywell Bull, there is present at either side of a given track P of a magnetic disc (an identical reasoning may equally be applied for magnetic tapes) having a "circular symmetry axis" Ax.sub.p, a zone of a width substantially equal to .delta. which contains magnetic data having the memory of the earlier state of the carrier, that is the state the carrier had, for example, before the track P had been recorded by means of a data write transducer associated with the magnetic disc. The value .delta. is the maximum limit of the accuracy of the system for positioning the write transducer opposite the magnetic disc, beyond which it is impossible to go.
By definition, the expression "immediate environment of the track P" denotes the total formed by the data of the zone of width .delta. and by the data of the tracks adjacent to the P' and P".
Consider for the moment a magnetoresistance transducer comprising two magnetoresistance elements positioned, for example, facing a magnetic disc.
The two magnetoresistances are then exposed, not only to the component normal to the magnetic disc of the magnetic leakage field of the magnetic area opposite which they are positioned, but equally--on the one hand to the resultant H.sub.envi of the magnetic leakage fields generated by the immediate environment of the track P"--on the other hand to the resultant H.sub.iv of the magnetic leakage fields generated by the magnetic areas situated on the track P at either side of the area opposite which the two magnetoresistances are positioned.
These two resultants H.sub.envi and H.sub.iv are the cause of a noise signal having the same frequency as the signal resulting from reading the magnetic leakage fields of the data of the different areas of a given track.
It is demonstrable that when the radial data density increases, the noise signal caused by the resultant H.sub.envi equally increases.
Similarly, when the linear data density increases, it is shown that the noise signal caused by the resultant H.sub.iv increases. It then becomes more difficult to detect any useful signal corresponding to a given data of a track of the carrier by contrast to the noise signals.
In existing practice, magnetic screening means formed by a set of thin laminations of preferably anisotropic magnetic material, which are coupled together and separated by non-magnetic layers, are placed at either side of the magnetoresistances so as to cancel the action of the resultant H.sub.iv on their output signal. The plane of each of the laminations is perpendicular to the record carrier and to the direction of travel of the tracks. The height of these screening means (their dimension measured perpendicular to the record carrier) greatly exceeds that of the magnetoresistant elements of the transducer. In the case in which the magnetic material of the screening means is anisotropic, the axis of difficult magnetisation of these means is oriented perpendicular to the magnetic carrier, so that the magnetic field lines generated by the areas which on the same track surround the magnetic area opposite to the magnetoresistance are not intercepted by the two magnetoresistance elements.
When the linear data density reaches a value of the order of 5000 inversion of magnetic flux per centimeter (which corresponds to 5000 changes in direction of the magnetic induction), this means that the length of each elementary magnetic area is of the order of 2 to 2.5 microns, the following factors intervene:
(1) The resultant H.sub.iv becomes substantial (of the order of the component H.sub.f of the magnetic leakage field):
(2) The distance between the magnetic screening means and the magnetoresistance elements becomes so small (of the order of one micron) that the magnetic coupling between the magnetoresistance elements and the screening means becomes substantial.
The consequence of the two actions cited above is that a sizeable mutual induction occurs between the magnetic screening means and the magnetoresistance elements which causes appreciable modification of the magnetization in the latter (magnetization greater than the linear data density). This sizeable mutual induction results in an interference signal which may destroy the data which it is wished to read, that is to say which may wholly neutralize the effect of the component H.sub.f of the magnetic leakage field on the two magnetoresistances.